TypeError: Unable to read $ auth property from undefined Vuejs

I am using vuejs 2 with laravel 5.4 which is integrated with it. I am trying to create an authentication service that uses laravel passport. I was able to login, but the problem is with vue js. I created a dedicated Auth package to handle the front end token. I am getting the error Cannot read property setToken from undefined

my auth package:

export default function (Vue){
Vue.auth = {
    // set token 
    setToken : (token , expires_in) =>{
    localStorage.setItem('token' , token);
    localStorage.setItem('expires_in' , expires_in)
    },
    // get token
    getToken : ()=>{

    },

    // destroy token


    // isAuthenticated
    isAuthenticated : ()=>{
    if(this.getToken())
    {
        return true
    }
    return false
    }
}
Object.defineProperties(Vue.prototype , {
    $auth : {
        get: ()=>{
        return Vue.auth
    }
    }
})
}

      

        
        
        
      

    

And this is my boot file:

       window._ = require('lodash');

    /**
     * We'll load jQuery and the Bootstrap jQuery plugin which provides support
     * for JavaScript based Bootstrap features such as modals and tabs. This
     * code may be modified to fit the specific needs of your application.
     */
      window.$ = window.jQuery = require('jquery');

require('bootstrap-sass');

/**
 * Vue is a modern JavaScript library for building interactive web interfaces
 * using reactive data binding and reusable components. Vue API is clean
 * and simple, leaving you to focus on building your next great project.
 */

window.Vue = require('vue');

import VueRouter from 'vue-router';

import Auth from './packages/auth/Auth.js';

Vue.use(VueRouter);
Vue.use(Auth);

/**
 * We'll load the axios HTTP library which allows us to easily issue requests
 * to our Laravel back-end. This library automatically handles sending the
 * CSRF token as a header based on the value of the "XSRF" token cookie.
 */

window.axios = require('axios');

window.axios.defaults.headers.common = {
    'X-Requested-With': 'XMLHttpRequest'
};

axios.defaults.baseURL = 'http://localhost/iAttendanceLaravel/public/';



/**
 * Echo exposes an expressive API for subscribing to channels and listening
 * for events that are broadcast by Laravel. Echo and event broadcasting
 * allows your team to easily build robust real-time web applications.
 */

// import Echo from 'laravel-echo'

// window.Pusher = require('pusher-js');

// window.Echo = new Echo({
//     broadcaster: 'pusher',
//     key: 'your-pusher-key'
// });

      

        
        
        
      

    

And this is finally my login method in the login.vue file:

  methods: {
        login: function () {

            var data = {
                client_id : 2,
                client_secret : '8WCDtW3wKeeNUBgwHviFoX7JmaVPU0HjFto9kwqv',
                grant_type     : 'password',
                username : this.loginForm.email,
                password : this.loginForm.password
            };

            axios.post('/oauth/token', data)
                .then(function (response) {
                    console.log(response);
                    self.$auth.setToken(response.body.access_token , response.body.expires_id + Date.now());
                })
                .catch(function (error) {
                    console.log(error);
                });

        },